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University of California Publications in Zoology [Vol. 12 



Pig. Q. Diagram illustrating possible explanation of presence of related 

 forms in same locality. 



located. The vertical lines represent geographical, the horizontal 

 and slanting lines physiological barriers. Thus we have 1, 2, 3, 4 

 and so on, contiguous geographic areas, each separated from the 

 other by some sort of geographic barrier. If A is the primitive 

 mammalian stock, then B may be regarded as a stock which acci- 

 dentally or otherwise has become geographically separated from it, 

 and finally has become differentiated into a separate species. The 

 continuation of this process of migration and differentiation may be 

 conceived of as resulting in species G, which has become so far dif- 

 ferentiated from the original stock A as not to be included in the 

 same subgenus with it. 



The ecologic niche b, occupied by species B, will be slightly dif- 

 ferent from niche a, in which the primitive stock is found. That 

 occupied by species C will differ slightly from that occupied by B. 

 This very slight difference in ecologic niche will be apparent up to 

 and including g, in which species G, which has attained a quite con- 

 siderable divergence, is found. 



Thus the series of niches, into which members of the subgenus 

 of which species G is representative will be able to enter, will be 

 somewhat different from the series to which members of the subgenus 

 of which species A is representative will have ready access. Thus it 

 may happen, through a further continuation of the process of 

 migration and differentiation, that species H to M, members of the 

 G subgenus, may arise, and there may even be a re-invasion, ou the 

 part of some member of the G group, as for instance M, into the 

 geographic area occupied by original stock A. But M will have 





